The present invention relates to an FM demodulation circuit, more specifically to an FM demodulation circuit using PLL circuit.
In recent years, the mobile vehicle communication system utilizing FM modulation has been widely employed. From the view point of miniaturization requirement for portability of the radiowave equipment, LSI technology has rapidly progressed. This tendency is also seen in FM demodulation circuit. The FM demodulation circuit using PLL has been popular with the progress of LSI technology and may be established by the process of C-MOS and the like suitable for LSI technology.
The conventional FM demodulation circuit using PLL will be described by reference to FIG.2. In FIG.2, an FM signal supplied from a signal input terminal 1 is inputted to a phase comparator 2 to detect a phase difference between the frequencies of the input signal from a comparison signal oscillator 3 and the FM signal. A comparison detection signal corresponding to the phase difference is sent to the comparison signal oscillator 3 to make a coincidence between the phase of the oscillation frequency signal of the comparison signal oscillator 3 and that of the FM signal, The comparison detection signal obtained in the phase comparator 2 is the DC voltage corresponding to the modulated frequency of the FM signal and is outputted from a demodulation signal output terminal 4 as the FM demodulation signal.
In such conventional FM demodulation circuit, it is required that the oscillation frequency of the comparison signal oscillator 3 is coincident with the frequency of the FM signal to be demodulated. However, if a frequency variable range of this oscillation deviates due to, for example, temperature rise, the frequencies will not be coincident owing to a limitation of the comparison detection signal (DC voltage) from the phase comparator 2, causing a distortion in the FM demodulation signal.
FIG.3 shows relationships between the comparison detection signal (corresponding to the oscillation frequency voltage) and the oscillation frequency for two conditions, i.e., the normal condition of the normal variable range of the oscillation and the deviation condition from the normal condition. In the normal condition as shown in FIG.3A, the accurate comparison detection voltage may be obtained depending upon the frequency of the FM signal to be demodulated. However, when the deviation occurs as shown in FIG. 3B, the frequencies are not coincident and the comparison detection signal reaches the limit value due to the nonlinear characteristics, causing the distortion in the comparison detection signal (demodulated signal).
More concretely, it is assumed now that the output voltage range generated from the phase comparator 2 is +1 to +4 V. Normally, when a power source voltage for exciting the phase comparator 2 is +5 V, the phase comparator 2 is capable of generating the output voltages of +1 V to +4 V.
Referring to FIG. 3A illustrating the normal characteristic at tile room temperature, the oscillation frequency varies from -16 kHz to +24 kHz with the frequency control input terminal voltage of +0 V to +5 V for the carrier frequency 455 kHz. When the modulation frequency range of the FM demodulation signal to be FM demodulated is -8 kHz to +8 kHz, the frequency control signal voltage (output voltage of the phase comparator 2) ranges from +1 V to +3 V. On the other hand, as shown in FIG. 3B, when the characteristic of the comparison signal oscillator 3 deviates due to the external condition change such as the temperature rise, tile voltage range of +0.5 V to +2.5 V is required as the frequency control signal for accurate response to the modulation frequency range of -8 kHz to +8 kHz.
However, since the phase comparator 3 can not generate the voltage lower than +1 V, the demodulation signal is maintained at +1 V for the frequency range from 455 kHz -4 kHz to 455 kHz -8 kHz of the signal to be FM demodulated, resulting in the demodulation signal distortion.
In order to solve this problem, it is required to adjust the respective oscillation frequency characteristic of the comparison signal oscillators. However, there has been no LSI circuit equipped with means for adjusting the respective deviations of the comparison signal oscillators.